The present invention relates to a method of checking a test circuit provided for testing logic circuits, more specifically to a method of checking connections between each of a plurality of circuit blocks and between each circuit block and a plurality of external terminals. Such a logic circuit and its testing method have been so far proposed that a test circuit is connected to a logic circuit having a plurality of circuit blocks to test the logic circuit for each circuit block. For instance, there exists such a method that a test circuit (e.g., a multiplexer) is connected between an input terminal of a circuit block and an external terminal or between an output terminal of a circuit block and an external terminal, and further an input signal is applied from the outside selectively for each block circuit, to observe or inspect an output signal thereof for each block circuit. In the prior art testing method, it has been possible to apply the input signal to each circuit block selectively from the outside or to transmit the output signal from each circuit block to the outside, by switching the connection relationship between the input and output of the multiplexer appropriately. However, it has been so far impossible to inspect whether there exists any erroneous connection between the circuit blocks and between the circuit block and the external terminal in the normal operation.
In summary, in the prior art logic circuit and its testing method, since it has been impossible to verify whether the connection relationship between the circuit blocks is correct or not in the normal operation, this verification has been so far dependent upon workers' visual inspection. As a result, there exists such a problem in that the erroneous connection between the circuit blocks are overlooked or a long test time is required.